This invention relates to circuits for stabilizing the output of lasers and, more particularly, to stabilizing the output of junction lasers to variations in the amplitude of drive current pulses.
As pointed out in U.S. Pat. No. 3,946,335 granted to B. C. DeLoach, Jr. and M. DiDomenico on Mar. 23, 1976, and assigned to the assignee hereof, the output power of a junction (diode) laser is known to vary as a function of temperature, natural aging and drive current. The latter is particularly important because of the ease with which catastrophic mirror damage occurs in junction lasers when the output power density exceeds the critical value. Because the catastrophic threshold can be exceeded as a result of spurious fluctuations in the amplitude of drive current pulses, it is important to reduce the sensitivity of the laser to such variations.
In the aforementioned patent, the output of a junction laser is stabilized by a negative feedback circuit in which a drive transistor is connected in common emitter configuration, the laser is connected between the collector and emitter, a photodetector is connected between the base and emitter and means couples a portion of the laser radiation to the photodetector. Photocurrent generated in the photodetector by the incident laser radiation reduces the base current, thus providing negative feedback. This circuit provides substantial immunity from changes in laser slope efficiency (to first order), threshold current, drive current and drive transistor gain. In addition, it is sufficiently fast acting to be capable of responding on a pulse-by-pulse basis in a digital system; that is, a system in which the drive current is a stream of encoded current pulses (PCM). On the other hand, this circuit does not lower the sensitivity of the laser to variations in amplitude of drive current pulses on a percentage overshoot basis which is most apt to be the case in a pulse circuit. That is, it is more likely that the percentage overshoot of pulse amplitude will be constant as the total current is changed rather than that the magnitude of the overshoot will be a constant. In addition, the D&D (DeLoach & DiDomenico) circuit improves sensitivity by lowering the slope of the L-I curve above threshold which presents two potential problems: (1) to get relatively high level light output high drive currents are required, but such high currents may not be within the capability of the drive circuit; and (2) the laser transition through threshold is more gradual so that the on-off (bistable) states are less clearly defined.
It is, therefore, an object of the present invention to provide all of the advantages attendant the stabilization circuit of D&D and, in addition, to reduce the sensitivity of the junction laser both to variations in the absolute amplitude of drive current pulses as well as variations on a percentage of overshoot basis.
It is another object of our invention to reduce the drive current required to generate a given light output level.
It is a further object of our invention to provide clearly defined bistable states for laser operation.